Conventional GFCI devices are designed to trip in response to the detection of a ground fault condition at an AC load. Generally, the ground fault condition results when a person comes into contact with the line side of the AC load and an earth ground at the same time, a situation which can result in serious injury. The GFCI device detects this condition by using a sensing transformer to detect an imbalance between the currents flowing in the line and neutral conductors of the AC supply, as will occur when some of the current on the line side is being diverted to ground. When such an imbalance is detected, a circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all power from the load. Many types of GFCI devices are capable of being tripped not only by contact between the line side of the AC load and ground, but also by a connection between the neutral side of the AC load and ground. The latter type of connection, which may result from a defective load or from improper wiring, is potentially dangerous because it can prevent a conventional GFCI device from tripping at the intended threshold level of differential current when a line-to-ground fault occurs.
GFCI devices may be connected to fuse boxes or circuit breaker panels to provide central protection for the AC wiring throughout a commercial or residential structure. More commonly, however, GFCI devices are incorporated into electrical receptacles that are designed for installation at various locations within a building. A typical receptacle configuration, as shown, for example, in U.S. Pat. No. 4,568,997, to Bienwald et al, includes test and reset pushbuttons and a lamp or light-emitting diode (LED) which indicates that the circuit is operating normally. When a ground fault occurs in the protected circuit, or when the test button is depressed, the GFCI device trips and an internal circuit breaker opens both sides of the AC line. The tripping of the circuit breaker causes the reset button to pop out and the LED to be extinguished, providing a visual indication that a ground fault has occurred. In order to reset the GFCI device, the reset button is depressed in order to close and latch the circuit breaker, and this also causes the LED to illuminate once again.
Although GFCI receptacles of the type described above can provide useful protection against electrical shock hazards, they are designed to trip only in response to a ground fault condition and not in response to other types of conditions which can also be dangerous. For example, although a broken neutral conductor in an AC power source will de-energize the load, the line side of the AC source is still energized and can create a shock hazard at the load. A conventional GFCI device will not detect this condition, since the broken neutral conductor disables the control circuitry within the GFCI device and renders it incapable of tripping. Similarly, when the electrical load is a power tool or some other type of power equipment that can cause injury when power is applied unexpectedly, the user may be endangered if power is removed (e.g., due to a circuit overload at a main fuse box or circuit breaker panel) and then restored while the equipment power switch is inadvertently left in the "on" position. Again, a conventional GFCI circuit provides no protection under these circumstances, because the power interruption does not trip the circuit breaker and does not prevent the circuit breaker from being reset even if it has already been tripped. In both of the situations mentioned, it would be desirable to employ a GFCI circuit which trips when the power supply is interrupted and cannot be reset until power is restored.
Portable GFCI devices have been designed for use in situations where the available AC power supply circuit does not include a central or receptacle-type GFCI device. These portable devices may be incorporated into line cords, extension cords or plug-in units, and are often used with power tools and other types of potentially hazardous power equipment at construction sites and the like. The need for a GFCI device which can be tripped by the absence of supply power is particularly great under these circumstances, not only because power equipment is more likely to be involved, but also because the connection of the GFCI device to the AC power receptacle can itself cause unexpected starting of the equipment if the power switch of the equipment has been left in the "on" position.
A number of GFCI circuits have been devised which are capable of being tripped and reset not only in response to a ground fault condition, but also in response to an undervoltage condition or a power supply interruption. Examples may be found in U.S. Pat. No. 3,548,259, to McDonald, and in U.S. Pat. No. 4,197,567, to Dietz et al. Although these circuits provide useful protection beyond that afforded by ordinary GFCI devices, they still rely on mechanical circuit breakers to trip in response to the undervoltage condition. Mechanical circuit breakers add undesirable complexity and expense to the GFCI circuit, and are also subject to failure due to the mechanical nature of the tripping and latching functions.
Newer types of GFCI devices employ relays, rather than circuit breakers or other types of mechanical latching devices, to interrupt the load power when a ground fault condition occurs. An electronic circuit controls the flow of current to the relay coil, and the relay contacts serve to open and close both sides of the AC line in response to the presence or absence of a ground fault condition. In these devices, only a simple momentary pushbutton switch is needed to perform the reset function, since the latching is performed electronically rather than mechanically. This results in a simpler, less expensive and more reliable device. Unfortunately, however, the nature of the circuit is such that it is set automatically when the GFCI device is initially connected to an AC power source, and after a power supply interruption. Ideally, it would be desirable to incorporate a manual set feature into a GFCI device of this type without relying on circuit breakers or other types of mechanical latching means.